Large quantities of liquid egg products are pasteurized each year to produce products sold in refrigerated form with limited shelf life. See qenerally. Egg Pasteurization Manual (USDA Agricultural Research Service 1969). More recently, the provision of techniques for ultrapasteurizing liquid egg products has led to considerable interest in marketing liquid egg products which have extended shelf lives under refrigerated conditions.
M. Hamid-Samimi et al., in IUFOST Symposium on Aseptic Processing & Packaging of Foods Proceedings. 229 (Symposium held September 9-12, 1985, in Tylosand, Sweden), concerns ultrapasteurizing liquid egg in which the egg is heated by contacting to a heated surface. See also U.S. Patent Application Serial No. 904,744, filed Sep. 8, 1986. A problem with this technology is the tendency of the heated surface to eventually foul.
U.S. Pat. No. 3,113,872 to Eynon Jones discloses a method of treating liquid egg products (shown schematically in FIG. 2 therein) which employs (a) a first heating step in which liquid egg is contacted to a heated surface, (b) a first vacuum evaporation and cooling step, (c) a second heating step in which liquid egg is contacted to steam, and (d) a second vacuum evaporation and cooling step. In the first heating step, the egg is heated to a temperature between 140.degree. and 144.degree. F. (Col. 9; lines 6-13). Between the first heating step and the first vacuum step, the egg is held for about 30 seconds at a temperature of about 143.degree. F. (Col. 9; lines 14-18). In the first vacuum step, liquid is held in a vacuum tank for three minutes and leaves the tank at a temperature of not less than 140.degree. F. (Col. 9; lines 34-56). In the second heating step, the egg is delivered to a steam infusion chamber where it is heated to a temperature of between 155.degree. and 165.degree. F. for about one second (Col. 10; lines 27-31 and 72-75). This second heating step is carried out under a vacuum (Col. 10; lines 18-21), with the egg being drawn by vacuum from the steam infusion chamber to the second vacuum chamber for the second vacuum step (Col. 10; lines 21-27). In the second vacuum step, the egg is held for about six seconds and cooled to 140.degree. F., and then removed for further cooling (Col. 11; lines 3-8 and 16-40). T. Sugihara et al., Food Technology 20, 1 (1966) discloses a method of treating liquid whole egg (shown schematically in FIG. 1 on page 4 thereof) which employs (a) a first heating step in which liquid egg is contacted to a heated surface, (b) a second heating step in which the liquid egg is contacted to steam, and (c) a vacuum evaporation and cooling step. In the first heating step, egg is heated to between 134.degree. and 149.degree. F. (Page 2, Col. 3). The egg is held for a time between 1 to 31/2 minutes, with a temperature drop generally not greater than 1.degree. F., between the first and second heating step. In the second heating step, the egg is heated by steam under vacuum to a temperature between 150.degree. and 170.degree. F. The egg is then drawn by vacuum into a vacuum chamber, where it is flash-cooled to about 127.degree. F. (Page 2, Col. 3 -Page 3, Col. 1).
Sugihara et al., at Page 4, Column 3 thereof, note that water gain in the treated egg from steam condensation during the steam heating step is offset by water loss during the vacuum cooling step. Thus, their process apparently did not appreciably affect the solids content of the treated egg. However, these authors go on to acknowledge that the loss of CO.sub.2 from the egg during vacuum cooling usually increased the pH of the egg from about 7.4 to about 7.8 to 7.9. Raising the pH of egg disadvantageously affects the sulfur chemistry of the egg, and can cause the egg to turn green in color.
L. Kline et al., Food Technology, 105 (Nov. 1965), at page 114, report a method of processing liquid egg white similar to the method of processing liquid whole egg disclosed by Sugihara et al., supra. A problem with Kline et al., Sugihara et al., and Eynon Jones is the difficulty of regulating a process in which the egg product is withdrawn from the bottom of a steam infusion chamber by vacuum after only a few seconds therein.